Remote emissions sensing systems (hereinafter “RES systems”) for detecting the concentrations of one or more gases in an exhaust plume of a motor vehicle are known. Conventional systems are configured to direct electromagnetic radiation through an exhaust plume of a passing vehicle, and monitor the intensity of the electromagnetic radiation received at a detector after the electromagnetic radiation has passed through the plume. As the electromagnetic radiation passes through the plume, gases within the plume may absorb the radiation within specific wavelength absorption bands that correspond to the particular gases present. Based on the amount of light transmitted and/or absorbed by the exhaust plume within the specific wavelength absorption bands, the concentrations of one or more of the gases may be determined.
Typically, to monitor the amount of radiation transmitted and/or absorbed within specific wavelength absorption bands, a conventional RES system may spatially separate the electromagnetic radiation into a spectrum and direct the spectrum incident on a sensor array, such as a diode array or other sensor array. A wavelength scale may be used to associate signals from the pixels (e.g., the individual diodes) with predictions of the wavelengths of radiation that may be received by the pixels, thereby enabling the intensities of radiation within particular wavelength bands to be determined. However, one source of inaccuracy in conventional systems may include a phenomenon known as “wavelength drift.” “Wavelength drift” is a phenomenon in which the spectrum of spatially arranged electromagnetic radiation incident on the sensor may wander such that the wavelength ranges of the electromagnetic radiation received by the sensor pixels vary over time. For example, ambient conditions (e.g., temperature, pressure, etc.), mechanical instability, and/or other factors may cause “wavelength drift” within the detector. As the portion of the spectrum incident on each of the pixels in the sensor array wanders, the association of the signals generated by the pixels with particular wavelengths, or wavelength ranges, by the wavelength scale may shift.
Current methods for correcting for “wavelength drift” in RES systems tend to be inefficient and/or inaccurate. For example, a typical approach for calibrating for “wavelength drift” may include inserting a mixture of gases with known concentrations into an optical path of electromagnetic radiation (e.g., using a sealed gas cell, or a “puff” of gas), and comparing the intensities of the received electromagnetic radiation across the electromagnetic wavelength spectrum with expected intensities based on the predetermined concentrations of the gases. If the expected intensities are shifted from the measured intensities along the wavelength spectrum, the wavelength scale may be shifted by an integer amount of pixels to correct for the shift. Such a method for calibrating for “wavelength drift” may be inefficient because the introduction of the mixture of gases of known concentrations into the optical path may require additional system capabilities (e.g., positioning a gas cell, providing a “puff” of gas, etc.). This method may also be inaccurate because it only enables calibration for “wavelength drift” that is at least as large as on pixel of the sensor array. In other words, current solutions for correcting for “wavelength drift” may not enable for corrections of “wavelength drift” smaller than a single pixel.
These and other drawbacks exist with known methods for correcting for “wavelength drift” in RES systems.